The present invention relates to demodulators able to restore a binary code with which an analog signal has been coded, by allocating to the latter two separate frequencies F.sub.a and F.sub.b, each corresponding to a given binary state of the code. Thus, such a demodulator receives an analog signal in the form of a succession of sinusoid trains, whose frequency is either F.sub.a or F.sub.b, and has to supply a succession of logic levels corresponding to the succession of frequencies reaching it. It is therefore a binary frequency modulated signal demodulator or frequency shift keying (F.S.K.) demodulator, meaning coding by frequency shift.
Such F.S.K. demodulators are more particularly used in telephony for transmitting a binary code by two frequencies (in practice 1300 and 2100 Hz) in the audible spectrum, which is compatible with the pass band of the telephone lines. These demodulators use switched capacitor filters and charge transfer filters.
Conversion to much higher frequencies is not possible in view of the frequency limitations of such filters. However, it is necessary to pass to very high coding frequencies F.sub.a and F.sub.b in the case when it is wished to transmit a high binary information flow rate. Thus, if the coding frequencies are 1300 and 2100 Hz, it is necessary to have a minimum of one signal cycle at 1300 Hz to recognize whether it is the frequency 1300 Hz and not the frequency 2100 Hz. Thus, the binary information flow rate is limited to a maximum of 13 bits/second.
A much higher flow rate is required for a large number of applications, such as satellite telecommunications for television signal transmission, multiplexed telephone communications, numerized sound, teletexts, etc. A typically desired flow rate is 2.048 megabits/second.
It would be possible to use inductor and capacitor filters, which can easily operate at high frequencies. However, these are in fact recursive filters, which therefore have a long and theoretically infinite pulse response and it is clear that if the information flow rate is high at the input, there is a significant risk of interference between two successive binary informations passing through the filters, this risk increasing as the filters become more selective. As the selectivity is made necessary by the proximity of the frequencies F.sub.a and F.sub.b to be discriminated, due to the fact that it is wished to reduce to the greatest possible extent the band width of the frequency modulation, a dead-end is reached if it is wished to obtain both a very high binary information flow rate and a limited width modulation.
In order to solve these different problems, the present invention proposes a filter for the demodulation of a signal modulated in frequency according to a binary code with two frequencies F.sub.a and F.sub.b (F.sub.b higher than F.sub.a), wherein the filter comprises on the surface of a piezoelectric crystal, two acoustic surface wave filtering units using interdigitated or split-finger electrode transducers deposited on the crystal, said units receiving in both cases analog signal to be demodulated and having a finite pulse response, whose duration is substantially equal to 0.5/(F.sub.b -F.sub.a), the first filtering unit having a frequency response curve with a very high relative attenuation at frequency F.sub.b and a relatively low attenuation at frequency F.sub.a, whilst the second filtering unit has a frequency response curve with a relatively high attenuation at frequency F.sub.a and a relatively low attenuation at frequency F.sub.b.
Throughout the remainder of the description and the claims, the term "relative" attenuation means the attenuation with respect to the maximum of the frequency response curve, said maximum having an attenuation taken as hypothetically equal to zero decibel.
The split-finger electrode transducers deposited on the crystal comprise, for each filtering unit, a transmitting transducer having a small number of interdigitated electrodes with a principle all the same length (unmodulated transducer) and a receiving transducer having a much larger number of interdigitated electrodes extending over a width (in the direction perpendicular to the electrodes) substantially equal to 0.5 v/(F.sub.b -F.sub.a), in which v is the propagation of the waves on the surface of the crystal, i.e. approximately 3000 m/s, as a function of the crystal used. The receiving transducer may be unmodulated (electrodes all of the same length) or modulated (electrodes of variable lengths) to modify the shape of the frequency response curve.
In particular, the receiving transducer can be modulated so as to have two groups of fingers separated by an empty space, the width of the complete transducer remaining substantially the same.
Obviously, due to the reciprocity, it is possible to interchange the characteristics of the transmitting and receiving transducers, whilst the overall pulse response, like the overall frequency response remain unchanged.
The spacing between two consecutive electrodes of the same potential is regular. This applies to the transmitting transducer and the receiving transducer of the same filtering unit. It is preferably substantially equal to v/(2F.sub.1 -F.sub.b) for the first filtering unit, for reasons which will be explained hereinafter, and v/(2F.sub.b -F.sub.a) for the second filtering unit.
According to a special embodiment, each filtering unit comprises a transmitting transducer and two receiving half-transducers facing the transmitting transducer. The two receiving half-transducers are identical, but one is moved back with respect to the other in the direction perpendicular to the transducer electrodes by a distance substantially equal to v/4F.sub.a for the first filtering unit and v/4F.sub.b for the second. In practice, these distances can be made approximately equal to a quarter of the spacing between two consecutive electrodes of the same potential of the receiving half-transducers. Thus, for each filtering unit, two half-filters are obtained, which supply identical signals in phase quadrature, which can be raised to the square (by passing through diodes) and added to bring about a particularly simple and effective detection.
The demodulation filter according to the invention has the advantage of considerable ease of manufacture, good stability and a possibility of high speed demodulation (low bit time--modulation band width product, compatible with a good propability of detection in the presence of noise).